Fabrication of electronic circuit elements using liquid deposition techniques is of profound interest as such techniques provide potentially low-cost alternatives to conventional mainstream amorphous silicon technologies for electronic applications such as thin-film transistors (TFTs), light-emitting diodes (LEDs), RFID tags and antennas, photovoltaics, etc. However the deposition and/or patterning of functional electrodes, pixel pads, and conductive traces, lines and tracks which meet the conductivity, processing, and cost requirements for practical applications have been a great challenge.
Solution-processable conductors are of great interest for use in such electronic applications. Metal nanoparticle-based inks represent a promising class of materials for printed electronics. However, most metal nanoparticles, such as silver and gold metal nanoparticles, require large molecular weight stabilizers to ensure proper solubility and stability in solution. These large molecular weight stabilizers inevitably raise the annealing temperatures of the metal nanoparticles above 200° C. in order to burn off the stabilizers, which temperatures are incompatible with most plastic substrates that the solution may be coated onto and can cause damage thereto.
Further, the use of lower molecular weight stabilizers can also be problematic, as smaller size stabilizers often do not provide desired solubility and often fail to effectively prevent coalescence or aggregation of the metal nanoparticles before use.
The printing of copper nanoparticles is currently being researched as a possible means to produce an electronic feature on a substrate because copper nanoparticle inks are cheap to produce. However, at present, copper nanoparticles are typically prepared by (1) electroplating copper ions onto an existing metal surface using corrosive and toxic reagents such as sodium hydroxide and cyanide or (2) various etched foil methods, which are both wasteful and incompatible with paper substrates. Furthermore, copper nanoparticle inks are often unstable and require an inert/reducing atmosphere during preparation and annealing to prevent the spontaneous oxidation to nonconductive copper (II) oxide or copper (I) oxide. Moreover, large copper nanoparticles (greater than 50 nm) require annealing temperatures greater than 1000° C., which is incompatible with most paper and plastic substrates.
Candidate liquid deposition materials thus include stabilized gold, silver and copper nanoparticles that are printed followed by high temperature sintering to anneal or weld together the particles to create a continuous, conductive line. Typically, a stabilizer molecule is attached to the surface of the particles, which is ‘burned off’ during the annealing step. Currently, however, metal nanoparticle inks suffer from several drawbacks. Gold and silver nanoparticle inks are very costly, and require high temperatures for annealing, which can pose a challenge for printing on paper and plastic substrates. Copper-based nanoparticle inks remain a research interest, due to their instability, and the requirement of an inert/reducing atmosphere during preparation and annealing to prevent spontaneous oxidation to non-conductive CuO or Cu2O.